The rapidly growing market of personal communication systems, radio medical implanted systems, and wireless hearing aids provides an increasing demand for more integrated and more efficient radio frequency (RF) integrated circuits (IC's). These IC's are required to operate with supply voltages under 2V and sometimes down to 1V with minimum current consumption at frequencies up to several GHz. Such applications typically contain a combination of several modules including a power amplifier, an oscillator, for example a voltage controlled oscillator (VCO), and modulator.
For example Class E power amplifier circuits are very suitable for high efficiency power amplification applications in the radio-frequency and microwave ranges. However, due to the inherent asymmetrical driving arrangement, existing Class E amplifier circuits suffer significant harmonic contents in the output voltage and current, and usually require substantial design efforts in achieving the desired load matching networks for applications requiring very low harmonic contents.
The basic Class E circuit is typically implemented using discrete components including a transistor, which is connected with an RFC to the supply voltage and to the load network. The load network is made up of a capacitor shunting the transistor and a series tuned inductor capacitor resonant circuit. The transistor is driven hard enough to act like a switch. The principle of Class E power amplifiers is to avoid by design the simultaneous existence of high voltage and high current in the switch, even in the case of a long switching time. That would imply 100% efficient conversion of dc to RF energy.
Frequency modulation is typically implemented via a varactor and is based on an LC-tank circuit. However, this requires additional discrete components to match the load network resulting in lower power efficiency. Typical solutions include using two identical resonant circuits, which encounters the same problem of matching inductors and capacitors, as well as using symmetrically driven push-pull Class E amplifier for high power applications.
A need therefore exist for providing an alternative oscillator design with frequency modulation capability, which seeks to address one or more of the above mentioned problems.